Transpiration cooling of high temperature components such as turbine blades is relatively well known in the art. This type of cooling generally features flow of a cooling fluid from a substrate interior with subsequent flow of the fluid through a porous outer layer of the substrate. This cools the blade by transporting heat within the porous outer layer and additionally acts to provide a boundary layer on the exterior of the blade surface, greatly mitigating contact between hot motive gases and the underlying substrate.
Transpiration cooled substrates generally consist of a main load-carrying strut and an external shell surrounding the strut which is constructed to allow the passage of coolant. A variety of materials and fabrication techniques have been applied and utilized as the porous, outer layer necessary for this type of transpiration cooling. In some cases, the porous outer layer is constructed by wrapping a porous skin sheathing such as wire cloth or ceramic tape around framework or the underlying substrate surfaces. See e.g. U.S. Pat. No. 3,067,982 to Wheeler issued Dec. 11, 1962; see also U.S. Pat. No. 3,706,508 to Moskowitz et al. issued Dec. 19, 1972; see also U.S. Pat. No. 4,311,433 to Bratton et al. issued Jan. 19, 1982; and see also U.S. Pat. No. 4,376,004 to Bratton et al. issued Mar. 8, 1983, among others. In other cases, transpiration strips are attached to surfaces comprised of metering holes. See e.g. U.S. Pat. No. 5,690,473 to Kercher issued Nov. 25, 1997, among others. In other cases, the load bearing struts are fabricated of porous, sintered metal materials, and the porosity of the load bearing member provides transpiration capabilities. See e.g. U.S. Pat. No. 3,647,316 to Moskowitz issued Mar. 7, 1972, among others. These methods have been effective at lower temperatures, however in the higher temperature environments where operating temperatures may be 1400° to 1700° C., the high temperature alloys typically used in these techniques oxidize, and the minute transpiration flow paths become plugged. Additionally, and significantly, at relatively high temperatures within about 300 C. degrees of the melting temperature of these high temperature alloys, continued sintering of the material tends to eliminate interconnected porosities and render the item non-permeable, negating the items original functionality.
In light of these issues, thermally resistant ceramic materials such as yttrium-stabilized zirconia (YSZ) have also been employed to function as outer, porous layers in transpiration cooling arrangements for components intended for high temperature service. Generally these ceramic materials are applied to underlying substrates featuring cooling fluid metering holes or channels, and the ceramic coating is applied by various known methods such as PVD or plasma spray processes. The ceramic coating is applied to be porous, so cooling air is able to penetrate the coating passages spread inside the ceramic layer before exiting the through porous surface of the layer. See e.g. U.S. Pat. No. 6,375,425 to Lee et al. issued Apr. 23, 2002; see also U.S. Pat. No. 6,511,762 to Lee et al. issued Jan. 28, 2003, among others. However, like ceramic TBC coatings generally, these porous ceramic coatings do not adhere well when applied directly to typical superalloys used as the substrates, and adhesion mainly by mechanical keying to a roughened bond coat is generally relied on. This provides limited tensile strength to the coating in service and additionally results in significant susceptibility to delamination-type failures driven by oxides thermally grown during service.
It would be advantageous to provide a method of fabricating a transpiration cooled substrate using a sintered material capable of metallurgically bonding with the underlying substrate, so that tensile strength stemming from the metallurgical bond rather than mechanical keying could be employed. It would additionally be advantageous if a high sintering resistance could be imparted to the sintered material as part of the fabrication process, so that interconnected porosities and permeability could be maintained during high temperature service.
It is generally understood that the sintering of metallic particles can be hampered by the presence of native oxide layers, such as alumina. See e.g., Munir, “Analytical treatment of the role of surface oxide layers in the sintering of metals,” Journal of Materials Science 14 (1979), among others. These oxide layers are tenacious and generally cannot be broken down or removed simply by heating during conventional sintering processes, and are generally dealt with through a variety of means during initial sintering, including reactive atmospheres, the addition of disrupting components such as Mg and Li, the use of liquid phase sintering, mechanical breakdown through friction techniques, ion bombardment treatments, and other means. See e.g. Xie et al., “Effect of Mg on the Sintering of Al—Mg Alloy Powders by Pulse Electric-Current Sintering Process,” Materials Transactions 45 (2004), among others. This retarding tendency of oxide layers has also been utilized to preserve resulting porosities during subsequent high temperature service, by intentionally promoting the formation of thick oxide layers on metallic particles prior to initial sintering. See U.S. Pat. No. 7,829,012 to Bischoff et al. issued Nov. 9, 2010. This approach reports to generate enhanced thermal resistance, however it would be preferable to provide a method whereby an oxide layer could be used to impart sintering resistance following the completion of an initial sintering phase, in order to minimize disruptions to the solid-state bonding between particles during the initial phase.
Provided here is a method and apparatus whereby an oxidized porous HTA layer is metallurgically bonded to a substrate through partial sintering followed by an oxidizing step to generate an adherent oxide layer between 20-500 nm thick on the exposed surfaces of the partially sintered particles. The partial sintering phase allows high-strength particle-to-particle and particle-to-substrate bonding to occur in the relative absence of an oxidation layer, and the intentional generation of an oxide layer over the resulting exposed surfaces greatly mitigates any additional sintering of the porous layer that would otherwise be expected in a high temperature service environments. Transpiration cooling capability is provided via micro-channel openings in the surface of the substrate, so that transpiration cooling flows may arise from the micro-channel to flow through the interconnected porosity of the oxidized porous layer. The methodology thereby provides an article for transpiration cooling whereby the porous layer and the substrate are metallurgically bonded for enhanced strength in service while the generated oxidation layer over the exposed surfaces acts to maintain effective porosity during service life.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.